Of course, what's happened here defies understanding. This evening we do know more what happened as it unfolded yesterday, how this gunman blazed his way into that school and abc's dan harris has the la. He reports now across town at the fire station. Dan, good evening. Reporter: David, good evening to you. As you know well having covered this story since the moment it broke, there have been so many contradictory and false reports about exactly what happened inside that school, but tonight, finally, we're get a fuller picture of those first, frantic moments. At around 9:30 a.M., The suspect armed to the teeth and coated in kevlar forced his way into the school by breaking the glass here at the front entrance. The chards still visible even from the air today. Units responding to sandy hook school. The front glass has been broken out of the school. They're unsure why. I can tell you it was believed he was not voluntarily let into the school at all, that he forced his way into the school. Reporter: Lanza had just driven the five miles from the home of his mother, nancy lanza, whom he had murdered. Once inside the school the rampage appears to have begun here near the office area. Among his first victims, principal dawn hochsprung. And the school psychologist. The principal of the school was running toward the shooter in order to protect her students, as was the school psychologist. Okay, all units, the individual continuing to hear what he believes to be gun shots. Reporter: Then lanza proceeded deep into the school where he killed those first-graders. The medical examiner saying today, that all of the children that he saw were hit multiple times. This probably is the worst that I have seen or the worst that I know of any of my colleagues have seen. Reporter: Police say the first 911 call came within minutes of lanza's arrival. One of the teachers helped them gut of a window. Reporter: Entering the school, evacuating the students and staff and searching the rooms until they found lanza dead, apparently by his own hand. And police told us today the entire disaster was over within, quote, mere minutes. So little time, david, and injcalculabel damage. All of those young lives lost so quickly. Dan harris has been here since the beginning. Of course the question that

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